Method for casting magnet bodies



Feb 8, 1966 R. 5,. QNRoY ETAL. 3,233,295

METHOD Foa CASTING MAGNET BODIES Filed Feb. '7, 1962 2 Sheets-Sheet 1 c :1S INVENToRs ROBERT 1 CONROY 44 42. JOHN J. JESMONT SAMUEL WEIMERSHEIIVIER MMT?, @mail M WW ATTORNEYS Feb. 8, 1966 R. L. coNRoY ETALv 3,233,295

METHOD FOR CASTING MAGNET BODIES Filed Feb. 7, 1962 2 Sheets-Sheet 2 ss (l BIN o H |N oERsTEDs (los) H :N oERsTEDs (|03) Z Q l,- @a Oi Z E 6 D J O U LL O E5 2 0. LLI O o l 3 4 5 e 7 e 9 |o MOLD PATTERN AREA METAL AREA F/G.5 F/G.6

ATTORNEYS United States Patent C 3,233,295 METHD FR CASTING MAGNET BODIES Robert L. Conroy, Murray Hill, .lohn J. JeSmOnt, Spots- Wood, and Samuel Weimersheimer, Rockaway, NJ., assignors to US. Magnet & Alloy Corporation, Bloomfield, NJ., a corporation of Delaware Filed Feb. 7, 1962, Ser. No. 171,664 8 Claims. (Cl. 22-212) The present invention relates to permanent magnets, and more particularly to a new and improved process for casting magnet bodies of improved unidireotionally oriented columnar grain structure and greatly increased magnetic properties from magnetically anisotropic alloys such as Alnico V `and Alnico VI.

It has long been recognized that the magnetically anisotropic properties of Alnico V alloys can be greatly improved by achieving long, substantially unidirectional grain growth within the castings, the resultant magnets being known as Alnico V-DG. By so doing, the subsequent heat-treatment of the cast bod-ies in a magnetic eld coinciding with the axis of the grain structure results in magnetic properties of relatively high strength. Although Alnico VI alloys exhibit anisotropic properties, heretofore it has not been considered Ipossible to Iachieve unidirectional grain growth.

It is the aim of the present invention to provide a new and improved process for casting permanent magnet bodies of magnetically anisotropic alloys and achieving a high degree of grain orientation and larger grain structure than heretofore possible to obtain greatly enhanced magnetically anisotropic properties.

Another aim is to provide a new and improved process for achieving directional columnanization in castings having a larger ratio of effective length to maximum transverse dimension (L/D ratio) than heretofore considered possible.

Still another aim is to provide a process for achieving directional grain columnanization in castings having shapes deviating .from rectangular hars and cylindrical rods.

A specific aim is to provide a new and improved process for casting permanent magnets of Alnico V alloys to obtain unidireot-ionally oriented columnar grain growth and greatly enhanced magnetically anisotropic properties.

Another specic aim is to provide a method for casting Alnico VI B alloys to obtain substantially unidirectionally oriented columnar grain growth and greatly enhanced magnetically anisotropic properties.

Other aims and advantages of our invention will be readily apparent from the following detailed description and claims and by reference to the attached drawings wherein:

FIG. l is a vertical section of a mold assembly constructed in accordance with the present invention for producing cylindrical magnet bodies;

FIG. 2 is a fragmentary plan View of the mold of FIG. 1;

FIG. 3 is a vertical section of an alternative embodiment of a mold assembly for producing horseshoe magnets;

FIG. 4 is a vertical section of another embodiment of mold assembly for producing multipole magnets;

FIG. 5 illustrates the macrostructure of a transverse section of an Aln-ico V magnet body cast in accordance with the present invention in molds similar to that illustrated in FIGS. 1 and 2;

FIG. 6 is a similiar illustration of the macrostructure of a transverse section of an Alnico VI B magnet body;

FIG. 7 illustrates the macrostructure of a horseshoe magnet cast from Alnico V alloy in the mold assembly of FIG. 3 in accordance with the present invention;

"ice

FIG. 8 illustrates the macrostructure of a multipole magnet cast from Alnico V alloy in the mold assembly of FIG. 4;

FIG. 9 shows the demagnetization curves of Alnico V magnets cast according to the present invention and that of the highest grade commercially available directionally grained Alnico V magnets prior to the present invention;

FIG. 10 shows demagnetization curve of directionally grained Alnico VI B magnets cast according to the present invention and that of conventional Alnico VI B magnets; and

FIG. 11 is a plot of the ratio of mold pattern area to metal area (mold chamber cross-sectional area) against depth of columnarization.

It has now been found that the foregoing and related objects can be attained by preheating a refractory mold assembly providing a reservoir above the mold chambers defining the magnet bodies to a temperature or" 1200 to 2700 F., and then introducing anisotropic magnet al-loy superheated lto a temperature at least 200 F. above the liquidus point but below the point at which twin grain formation occurs into the mold assembly in an amount sufficient to lill said mold chambers and provide a body of superheated alloy in the reservoir. The mold and metal temperatures are coordinated to provide a steep thermal gradient so that the susperheat and heat of solidification of the alloy in the mold chambers are extracted substantially entirely from a chill member at the bottom of the mold assembly lto obtain a unidi-rectionally oriented columnar grain structure.

As will be pointed out in detail hereinafter, the various conditions of temperatures of metal and of size and spacing of the mold chambers must be closely controlled to achieve the desired extraction of the superheat and the heat of solidiication through the chill member at the bottom of the mold assembly with minimization of transverse heat loss. Generally, the mold assembly is preheated to temperature `of 1200 to 2700o F. and preferably 1800 to 2700 F., and the metal is superheated at least 200 F. above the liquidus point and preferably 400 thereabove.

By the present invention, it has been possible not only to greatly enhance the magnetic anisotropy of Alnico V alloys but also to permit directional orientation of Alnico VI B alloys which heretofore has not been considered possible.

The relationship of the mold cavities or chambers to the mold pattern area, which is the area in which the pattern of mold chambers is located and exclusive of the reinforcement therearound, is quite critical to achieving the desired result. Basically, the mold material should be kept to a minimum commensurate with maintaining proper dimensioning and strength at the elevated temperatures. Accordingly, molds produced from refractory material bonded by alkali metal silicate or other high temperature refractory binders are utilized, which molds will maintain strength and dimensioning at the temperature of the preheat.

The terminology cross-sectional area of the mold chambers as used herein refers to the area of the horizontal cross-section taken through the mold chamber cavity and to the sum of such cross-sectional areas in any given mold.

The terminology cross-sectional area of the mold pattern portion as used herein refers to the area of the cross-section of that portion of the mold having its margins defined by the outermost portions of the outermost mold cavities in any given mold and including the area of the mold cavities therewithin, but not including the reinforcement area of the mold pattern which extends about the outer margins of the mold pattern, i.e., the outer peripheral portion.

As shown in FIG. 1l, the ratio of the mold pattern area to mold chamber cross-sectional area should be less than about 3:1 in order to obtain columnarization substantially throughout the height of the magnet. To minimize the transverse heat loss still further, the reinforcement area of the mold surrounding the mold pattern area should also be minimized with about 1/2 to 1 inch being satisfactory dependent upon the overall size of the mold. At a ratio of greater than 3:1, the depth of columnarization rapidly falls off as shown in FIG. 1l.

The thermal gradient relationship of the superheated metal and of the preheated molds is critical in that the chilling of the metal in the mold chambers must take place substantially entirely through the chill member to produce the directional columnarization, and the superheated metal in the reservoir above the mold chambers must ensure a steep thermal gradient throughout the mold chamber while avoiding substantial transverse heat loss during the chilling. Accordingly, the ratio of the effective length to the maximum transverse dimension of the mold chambers is also a significant factor for ensuring the necessary steep thermal gradient Without appreciable transverse heat loss during chilling of the metal throughout the effective length of the mold chamber. However, by the present invention, it has been found that unidirectionally oriented magnets can be produced at an effective length to diameter (or maximum transverse dimension) ratio of up to :1, although the preferred ratio is less than about 6:1.

The term steep thermal gradient as used herein refers to a large difference between the temperatures of the solidifying metal and the molten metal spaced upwardly therefrom.

It has been found that the several factors are correlated in the following yformula within the method of the Present invention:

wherein W--ratio of effective length of the mold chamber to maximum transverse dimension `(L/ D).

k1='2.1 for cylinders and plates, 1.9 for bars `and rectangles MB Superhcat=g(tb-t,) and 200 F.

C=Specific heat=0.18 (B.t.u./lb. F.)

\=Latent heat of fusion=i120 (Btu/lb. F.)

tbzTemperature of the superheated magnet alloy rs=Liquidus temperature of the magnet alloy tm=Temperature of preheat for the molds and 1200 to Since the thermal factors correlated by the above equation establish minimum conditions for substantially complete columnarization, the L/D ratio can be less than the factor resultant from the right hand portion of the equation.

As will be readily apparent to those skilled in the art, the liquidus temperature will vary with the composition 0f the magnet alloys. By proper relationship of the thermal factors, the present invention has been found not only highly effective with Alnico V alloys containing 6 to 11 percent by weight aluminum, 16 to 30 percent by weight cobalt, 12 to 20 percent by weight nickel, up to 7 percent by weight copper and the remainder principally iron, but also with Alnico VI B alloys additionally containing up to 2 percent by weight titanium. Generally, the liquidus temperature of Alnico V alloys is about 2680" F. and that of Alnico VI B alloys is about 2700 F. In practicing the present invention, Alnico V alloys should be superheated to a temperature of 2880 to 4 3450 F. and preferably 3200 to 3450" F. Alnico Vl E alloys should be superheated to temperatures of 2900 to 3500 F. and preferably 3200 to 3500 F. The maximuni temperature of the superheat is determined by the point at which twin grain formation will occur in the alloy.

The term magnet alloy as used herein refers to ferrous alloys capable of developing a high degree of anisotropy during heat-treatment in a magnetic field coinciding with the axis of columnarization and particularly to ferrous base alloys containing aluminum, nickel, cobalt and iron known as Alnico V and VI alloys.

The term superheated as used herein refers to magnet alloys which are heated to temperatures at least 200 F. above the liquidus temperature but below the temperature at which twin grain formation occurs, which temperature is sufficient to permit the extraction of the heat of the molten metal substantially entirely by the chill member to enable the columnar grain growth in combination with preheated refractory molds and process of the present invention.

For a more specific understanding of the present invention, reference is made to FIGS. 1-4 of the attached drawings wherein mold assemblies for practicing the present invention are illustrated. Referring first to FIGS. 1 and 2, a typical -mold assembly for casting cylindrical magnets is illustrated. Assembled on top of a metal chill plate 2 is a refractory mold 4 having a plurality of closely spaced mold chambers 6 extending upwardly from the chill member 2 and terminating inwardly of the upper face of the mold. Extending upwardly from the upper end of the mold chambers 6 to the upper face of the mold are gate passages 8 of lesser cross section than the mold chamber 6. To distribute metal into the mold chambers 6, a riser 10 having an enclosed chamber or reservoir 12 and a funnel 14, both fabricated from refractory material, are placed on top of the mold 6 and secured in place by a high-temperature adhesive (not shown).

Initially, the assembly of mold 6, riser 10 and funnel 14 are preheated to a temperature of about 1200 to 2700 F., and preferably 1800 to 2700 F., and then the assembly is placed upon the chill member 2. The superheated metal is then introduced into the funnel `14 to lill up the mold chambers 6, the gates 8 and to extend up into and preferably fill the reservoir or chamber 12 to produce a reservoir of superheated metal above the mold chambers.

The molten metal in the mold chambers 6 is rapidly chilled by extraction of heat through the chill member 2 until a colu-mnar grain structure is produced throughout the height of the mold chamber since the body of molten metal in the gate passages 8 and reservoir 12 maintains a steep thermal gradient during chilling of the metal within the mold chambers 6 and provid-es a reservoir of heat for the assembly to prevent premature chilling of the metal at the upper end of the mold chamber through transverse heat loss. After chilling and cooling, the mold assembly is readily disassembled by striking with a mallet and the solidified metal will fracture readily in the gate passages 8 due to the brittleness of the alloy and thus separate the desired cylindrical castings.

Indicative of the highly undirectionaly grained structures produced by the present invention in cylindrical or bar magnets are FIGS. 5 and 6 which show, respectively, the grain structure of transverse sections of Alnico V and Alnico VI B magnets produced in the mold of FIGS. l-2. The grains are columnar and oriented in the direc-- tion which can then be heat-treated in a magnetic field parallel thereto with resultant greatly enhanced magnetic anisotropy.

Referring now to FIGS. 3 and 7, the present invention enables the casting of irregularly shaped magnets such as the illustrated horseshoe shapes which evidence colurnnar grain structure throughout substantially the entire body as shown in FIG. 7. More particularly, the mold 20 has a plurality of horseshoe-shaped mold cavities 22 having the legs thereof extending upwardly from the steel chill plate 24. The cavities 22 terminate inwardly of the upper face of the mold, but gate passages 26 extend from the center of the bridge portion of the cavity to the upper surface of the mold. The riser 28 provides a reservoir chamber 30 by which molten alloy introduced into the funnel 32 is distributed into the gate passages 26 and thereby into the mold cavities 22. As shown, the molten alioy is introduced in an amount sufficient to provide a body of the superheated alloy above the mold chambers.

The mold assembly illustrated in FIG. 4 is utilized for producing multipole magnets which similarly exhibit columnar grain structure not only throughout the legs of the magnet but also through the bridge portions, as illustrated in FIG. 8. It has been found that the columnarized grains in the leg portions which cirectly communicate with the chill plate produce parallel columnarization of the metal in the bridge portions. As shown, the mold assembly in FIG. 4 similarly has a mold 40 with a plurality of E-shaped mold cavities 42 having their leg portions extending upwardly from the chill plate 44.v Gate passages 46 extend upwardly from the center of the mold cavities 42 and communicate with the reservoir chamber 48 of the riser 50 to distribute metal thereinto.

The molds, riser and funnel are fabricated of refractory material which will maintain its strength and dimensioning at the preheating temperatures of 1200 to 2700 F.. In practice, metallic oxides such as aluminum oxide, forsterite,zircon and mullite have proven advantageous when bonded by alkali metal silicates. Other bonding agents which have been used in mold production are oxychlorides, phosphates, organic silicates and metallo-organic compounds.

The chill member may be a metal plate fabricated from a high temperature resistance and corrosion resistant alloy and may utilize a coolant medium such as water, liquid metal or gasses, to aid in heat dissipation if so desired.

`As specilic examples of alloys which have proven particularly advantageous in the practice of the present invention are the following:

ALNICO V Preferred Specific Com- Element Range. Percent position.

by Weight Percent by Weight.

Aluminum 8. -8. 8 8. 3 Nickel 13. O-14. 0 13. 3 23. -25. 0 24. 5 1. 5-3. 5 2. 9 0. -0. 5 0. 2 0` 154). 5 G. 3

Norm-Remainder principally iron with minor impurities.

ALNICO VI Preferred Specific Com- Element Range, Percent position,

by Weight Percent by Weight 8. 0-8. 5 S. 5 14. 7-15. 5 15. 5 23. 5- 5.0 24. 0 2. 5-3. 5 3.0 0. 5-1. 5 0. 8 Titanium 0. 5-1. 5 1. 1

NOTE-Remainder principally iron with minor imp urities.

Indicative of the etiicacy of the present invention are the following specic examples wherein columnar grain structure and `greatly enhanced magnetic properties were obtained in Alnico V and Alnico VI B alloys:

Example one An assembly of a refractory mold, riser and pouring funnel was prepared and preheated to a temperature of 2000 F. The refractory mold assembly was similar to that illustrated in FIG. 1 of the attached drawing. The mold chambers were 0.759 inch in diameter and 0.522 inch in length, and the ratio of mold chamber cross-sectional area to total mold pattern area was about 1:2. The preheated mold assembly was placed upon a metal chill plate l inch in thickness.

An alloy containing 8.3 percent by weight aluminum, 13.3 percent by weight nickel, 24.5 percent by weight cobalt, 2.9 percent by weight copper, 0.2 percent by weight titanium, 0.3 percent by weight columbium and the remainder principally iron (with carbon and silicon impurities of less than 0.12 percent by weight) was superheated to a temperature of about 3350 F. and poured into the mold assembly to till substantially the entire reservoir chamber provided by the riser. Upon inspection of a transverse section of the chilled castings, they were found to be unidirectionally grained and to have only ten columnar grains.

The castings were then normalized at about 1700 F. for one-half hour and cooled in a magnetic field parallel to the axis of grain orientation for about fifteen minutes. The magnet bodies were then subjected to coercive aging at a temperature of about 1100 F. for about two hours and at about 1025 F. for twenty hours.

The magnets had a residual flux density (Br) of 13,400 gausses, a coercive force (Hc) of 810 oersteds and a maximum energy product (BHmax) of V 8.1 l06 gaussoersteds.

Example two A mold assembly similar to that in Example one was prepared having mold chambers 2 inches in diameter and 2 inches in length and using a mold chamber to mold pattern area ratio of about 2:1. The refractory mold assembly was initially preheated to a temperature of about 2000 F.

An alloy containing 8.5 percent by Weight aluminum, 13.0 percent by weight nickel, 23.8 percent by weight cobalt, 2.6 percent by Weight copper, 0.3 percent by Weight titanium, 0.2 percent by weight columbium and the remainder principally iron was superheated to a temperature of about 3450" F. and poured into the mold assembly in an amount suiicient to fill the reservoir chamber in the riser. Upon inspection of a transverse section, the castings were found to be unididectionally grained and to have only about forty columnar grains.

The castings were then heat-treated similarly to those in Example one. The magnets had a residual flux density (B1.) of 13,200 gausses, a coercive force (Hc) of 810 oersteds, and a maximum energy product (BHmaX) of 7.6 l06 gauss-oersteds.

Example three A mold assembly similar to that in Example One was prepared having mold chambers 1/2 inch in diameter and 2 inches in length and utilizing a mold pattern area to mold chamber cross-sectional area ratio of slightly less than 3:1, and was preheated to a temperature of 2200 F,

An alloy containing 8.5 percent by weight aluminum, 15.5 percent =by weight nickel, 24.3 percent by Weight cobalt, 3.0 percent by weight copper, 1.1 percent by weight titanium, 0.8 percent by weight columbium and the remainder principally iron was superheated to a temperature of about 3400 F. and poured into the refractory mold assembly in an amount sufficient to fill the reservoir chamber of the riser. Upon inspection of a transverse section, the castings were found to be substantially unidirectionally grained and to have only ninety grains at the chill face diminishing to sixty grains at the opposite face.

The castings were then normalized at about 1670" F. for about one-half hour and cooled in a magnet field parallel to the axis of grain orientation for about ten minutes. The magnet bodies were then subjected to coercive aging at atemperature of about 1225 F. for two hours and at about 1030 F. for twenty-four hours followed by cooling to room temperature.

The magnets thus produced were found to have a residual flux density (Br) of 10.600 gausses, a coercive force (Hc) of 930 oersteds and a maximum energy product (BHmw) of 5.2 G gauss-oersteds.

Example four A re-fractory mold assembly similar to that of Example One was prepared having mold chambers 0.875 inch in diameter and 3 inches in length and utilizing a mold pattern area to mold chamber cross-sectional area ratio of about 2:1. The refractory mold assembly was preheated toa temperature of about 1950 F.

An alloy containing 8.60 percent by weight aluminum, 15.25 percent by weight nickel, 23.5 percent by Weight cobalt, 2.9` percent by weight copper, 1.3 percent by weight titanium, 0.9 percent by weight columbium and the remainder principally iron (with impurities consisting mainly of silicon and carbon constituting less than 0-.12 percent by weight) was superheated to a temperature of about 3500 F. and poured into the mold assembly in an amount suicient to fill the reservoir chamber of the riser. Upon an inspection of a transverse section, the castings were found toH be substantially unidirectionally grained and to have only eighty grains at the chill face dimensioning to forty grains at the opposite face.

The castings were then treated similarly to those in Example Three. The resultant magnets had a residual ilux density (Br) of 10,300 gausses, a coercive force (Hc) of 950 oersteds and a maximum energy product (BI-Imax) of 5.6 106 gauss-oersteds.

Example five A mold assembly similar to that in Example One was prepared havingy mold chambers 0.50 inch in diameter and.

4.0 inches in length and utilizing a mold pattern area to mold chamber cross-sectional area ratio of 2:1, and was preheated to a temperature of about 2150 F.

An alloy containing 8.2-percent by weight aluminum, 13.2 percent by weight nickel, 24.4 percent by. weight cobalt, 2.9 percent by weight copper, 0.3 percentby weight titanium, 0.3 percent by weight columbium and the re.- mainder principally iron was superheated to a temperature of about 3300" F. and poured into the refractory mold assembly in an amount suflicient to till the reservoir chamber of the riser. l Upon inspection of a transverse section, the castings were found to be substantially unidirectionally grained.

The castings were then heat-treated similarly to those in Example One. The magnets had a residual ilux density (Br) of 13,500 gausses, a coercive force (Hc) of 870 oersteds and a maximum energy product (BHmaX) of 7.9)(106 gauss-oersteds.

Example six A mold assembly similar to that in Example One was prepared having mold chambers 1.0 inch in diameter and 0.75 inch in length and utilizing a mold pattern area to mold chamber cross-sectional area ratio of 1.8:1 and was preheated to a temperature of about 1400 F.

An alloy containing 8.3 percent by weight aluminum, 13.2 percent by weight nickel, 24.5 percent by weight cobalt, 2.9 percent by weight copper, 0.3 percent by weight titanium, 0.3 percent by weight columb-ium and the remainder principally iron was superheated to a temperature of about 3300" F. and poured into the refractory mold assembly in an amount sufficient to ll the reservoir chamber of the riser. Upon inspection of a transverse section, the castings were found t0 be SubStantially unidirectionally grained.

The castings were then heat-treated similarly to those in Example One. The magnets had a residual llux density (Br) of 13,400 gausses, a coercive force (Hc) of 760 oersteds and a maximum energy product (Bf-Imax) of 6.9 106 gauss-oersteds.

As will be evident from the foregoing detailed specification and claims, the 'present invention enables the casting of magnets having a highly dir-ectionally oriented columnar grain structure even in irregular and multipol-e shapes and even when using alloys other than Alnico V-DG.

We claim:

1. The method of casting unidirectional grained magnet bodies of an anisotropic ferrous magnet alloy comprising: providing a chill member and a refractory mold disposed thereon having a moid portion with a plurality of mold chambers therein extending vertically upwardly from said chill member and a portion providing a substantially closed reservoir chamber spanning the area above and communicating through restricted openings in the upper ends of said mold chambers with said mold chambers, said mold portion having a mold pattern portion defined by the periphery of the outermost mold chambers and a reinforcing portion extending thereabout with a ratio of mold chamber cross-sectional area to the area of mold pattern portion of greater than about 1:3 preheating the assembled mold and reservoir to a temperature of about 1200 to 2700 F.; pouring thru said reservoir into said mold assembly anisotropic ferrous magnet alloy superheated to a temperature at least 200 F. above the liquidus point but below the point at which twin grain formation occurs in an amount suicient to ll said mold chambers and provide a reservoir of superheated alloy above said mold chambers substantially closed to atmospheric heat losses, said mold and metal temperatures being selected to provide a steep thermal gradient during extraction of the superheat and heat of solidiication of the alloy in said mold chambers while minimizing transverse heat loss therefrom, said reservoir 0f superheated alloy in said reservoir chamber providing a reservoir of heat for the metal in said mold cavities to avoid premature chilling thereof and to enable solidification to occur through extraction of the heat from the molten metalv in said mold chambers through said chill member; and allowing said alloy to' solidify in said mold chambers with the heat thereof being substantially entirely extracted frorn the bottom of said chambers by said chill member to produce a substantially unidirectionally grained structure.

2. The method in accordance with claim 1 wherein said anisotropic ferrous magnet alloy contains 6.0 to 11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel, 16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by weight copper and the remainder principally iron, and wherein said alloy is superheated to a temperature of 2880 to 3450 F.

3. The method in accordance with claim 1 wherein said anisotropic ferrous magnet alloy contains 6.0 to 11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel, 16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by weight copper, 1.0 to 2.0 percent by weight titanium, and the remainder principally iron, and wherein said alloy is superheated to a temperature of 3300 to 3500 F.

4. The method of casting unidirectionally grained magnet bodies of an anisotropic ferrous magnet alloy comprising: providing an assembly comprising a chill member, a refractory mold thereon having a plurality of mold chambers therein extending vertically upwardly from said chill member and a super-posed riser member for distributing metal into said mold chambers and providing a substantially closed reservoir .spanning the area above and communicating with said mold chambers thru restricted openings in the upper ends of said mold chambers, said mold having a mold pattern area defined by the periphery of the outermost mold cavities and a reinforcing portion extending thereabout with a ratio of mold cavity cross-sectional area to total mold pattern area of greater than about 1:3, preheating the assembled mold and reservoir to a temperature of about 1200 to 2700 F.; introducing into said riser member anisotropic ferrous magnet alloy superheated to a temperature at least 200 F. above the liquidus point but below the point at which twin grain formation occurs and in an amount suicient to till said mold chambers and provide a reservoir of superheated alloy in said riser member above said mold chambers substantially closed to atmospheric heat losses, said mold and metal temperatures being selected to provide a steep thermal gradient during extraction of the superheat and heat of solidieation of the alloy in said mold chambers while minimizing transverse heat loss therefrom, said reservoir of superheated metal providing a reservoir of heat for the metal in said mold cavities to avoid premature chilling thereof and to enable solidiication to occur through extraction of the heat from the molten metal in said mold chambers through said chill member; and allowing said alloy to solidify in said mold chambers with the heat thereof being substantially entirely extracted from the bottom of said chambers by said chill member to produce a unidirectionally grained structure.

5. The method in accordance with claim 4 wherein said anisotropic ferrous magnet alloy contains 6.0 to 11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel, 16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by Weight copper and the remainder principally iron and wherein said alloy is superheated t a temperature of 2880 to 3450 F.

6. The method of casting unidirectionally grained magnet bodies of an anisotropic ferrous magnet alloy comprising: providing a chill member and a refractory mold assembly thereon having a mold portion disposed thereon with a plurality of mold chambers therein extending vertically upwardly from said chill member and a portion providing a substantially closed reservoir spanning the area above and communicating with said mold chambers, said mold portion having a mold pattern area dened by the periphery of the outermost mold cavities and a reinforcing portion extending thereabout with a ratio of mold chamber cross-sectional area to total mold pattern area of greater than about 1:3, preheating the assembled mold and reservoir to a temperature of about 1200 to 2700 F.; pouring thru said reservoir and into said reservoir and mold assembly anisotropic ferrous magnet alloy superheated to a temperature at least 200 F. above the liquidus point but below the point at which twin grain formation occurs and in an amount sufficient to ll said mold chambers and provide a reservoir of superheated metal above said mold chambers substantially closed to atmospheric heat losses, said reservoir of superheated metal providing a reservoir of heat for the metal in said mold cavities to avoid premature chilling thereof and to enable solidication to occur through extraction of the heat from the molten metal in said mold chambers through said chill member said mold and metal temperatures being selected to ensure a unidirectional grain producing gradient according to the following formula:

W= 0f (k1-i-k2) (MB) (MM) wherein W=ratio of effective length of the mold chamber to maximum transverse dimension (L/D) k1=2.1 for cylinders and plates, 1.9 for bars and rectangles MB=Superheat =g (tb -t,) and 200 F.

C=Specific heat=0.18 (B.t.u./lb. F.)

)\=Latent heat of fusion=l20 (B.t.u./lb. F.)

b=Temperature of the superheated magnet alloy ts=Liquidus temperature of the magnet alloy tm=Temperature of preheat for the molds and 1200 to and allowing said alloy to solidify in said mold chambers with the heat thereof being substantially entirely extracted from the bottom of said chambers by said chill member to produce a substantially unidirectionally grained structure.

"l. The method in accordance with claim 6 wherein said anisotropic ferrous magnet alloy contains 6.0 to 11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel, 16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by weight copper and the remainder principally iron and wherein said alloy is superheated to a temperature of 3200 to 3450 F.

8. The method in accordance with claim 6 wherein said anisotropic ferrous magnet alloy contains 6.0 to 11.0 percent by weight aluminum, 12.0 to 20.0 percent by weight nickel, 16.0 to 30.0 percent by weight cobalt, up to 7.0 percent by weight copper, 1.0 to 2.0 percent by weight titanium and the remainder principally iron, and wherein said alloy is superheated to a temperature of 3200 to 3500 F.

References Cited by the Examiner UNITED STATES PATENTS 1,850,477 3/1932 Roth 22-212 2,255,546 9/1941 Hansen 22-130 2,578,407 12/ 1951 Ebling 22-213 2,594,998 4/ 1952 Rocco 22-212 2,891,883 6/1959 Howe 22-212 2,940,142 6/1960 Wells et al. 22-130 FOREIGN PATENTS 527,174 9/ 1954 Belgium.

652,022 4/ 1951 Great Britain.

684,522 12/ 1952 Great Britain.

743,635 1/ 1956 Great Britain.

I. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, MICHAEL B. BRINDISI,

Examiners. 

1. THE METHOD OF CASTING UNIDIRECTIONAL GRAINED MAGNET BODIES OF AN ANISOTROPIC FERROUS MAGNET ALLOY COMPRISING: PROVIDING A CHILL MEMBER AND A REFRACTORY MOLD DISPOSED THEREON HAVING A MOLD PORTION WITH A PLURALITY OF MOLD CHAMBERS THEREIN EXTENDING VERTICALLY UPWARDLY FROM SAID CHILL MEMBER AND A PORTION PROVIDING A SUBSTANTIALLY CLOSED RESERVOIR CHAMBER SPANNING THE AREA ABOVE THE COMMUNICATING THROUGH RESTRICTED OPENINGS IN THE UPPER ENDS OF SAID MOLD CHAMBERS WITH SAID MOLD CHAMBERS, SAID MOLD PORTION HAVING A MOLD PATTERN PORTION DEFINED BY THE PERIPHERY OF THE OUTERMOST MOLD CHAMBERS AND A REINFORCING PORTION EXTENDING THEREABOUT WITH A RATIO OF MOLD CHAMBER CROSS-SECTIONAL AREA TO THE AREA OF MOLD PATTERN PORTION OF GREATER THAN ABOUT 1:3 PREHEATING THE ASSEMBLED MOLD AND RESERVOIR TO A TEMPERATURE OF ABOUT 1200 TO 2700*F.; POURING THRU SAID RESERVOIR INTO SAID MOLD ASSEMBLY ANISOTROPIC FERROUS MAGNET ALLOY SUPERHEATED TO A TEMPERATURE AT LEAST 200* F. ABOVE THE LIQUIDUS POINT BUT BELOW THE POINT AT WHICH TWIN GRAIN FORMATION OCCURS IN AN AMOUNT SUFFICIENT TO FILL SAID MOLD CHAMBERS AND PROVIDE A RESERVOIR OF SUPERHEATED ALLOY ABOVE SAID MOLD CHAMBERS SUBSTANTIALLY CLOSED TO ATMOSPHERIC HEAT LOSSES, SAID MOLD AND METAL TEMPERATURES BEING SELECTED TO PROVIDE A STEEP THERMAL GRADIENT DURING EXTRACTION OF THE SUPERHEAT AND HEAT OF SOLIDIFICATION OF THE ALLOY IN SAID MOLD CHAMBERS WHILE MINIMIZING TRANSVERSE HEAT LOSS THEREFROM, SAID RESERVOIR OF SUPERHEATED ALLOY IN SAID RESERVOIR CHAMBER PROVIDING A RESERVOIR OF HEAT FOR THE METAL IN SAID MOLD CAVITIES TO AVOID PREMATURE CHILLING THEREOF AND TO ENABLE SOLIDIFICATION TO OCCUR THROUGH EXTRACTION OF THE HEAT FROM THE MOLTEN METAL IN SAID MOLD CHAMBERS THROUGH SAID CHILL MEMBER; AND ALLOWING SAID ALLOY TO SOLIDIFY IN SAID MOLD CHAMBERS WITH THE HEAT THEREOF BEING SUBSTANTIALLY ENTIRELY EXTRACTED FROM THE BOTTOM OF SAID CHAMBERS BY SAID CHILL MEMBER TO PRODUCE A SUBSTANTIALLY UNIDIRECTIONALLY GRAINED STRUCTURE. 